Aircraft, particularly military aircraft, may carry microwave communications equipment that enables high-speed, beyond line of sight satellite communications. In the case of fixed-wing aircraft, there are many locations on an upper surface of the aircraft where a microwave antenna may be mounted to communicate with visible satellites with a clear line of sight.
On the other hand, mounting a microwave antenna on an upper surface of a helicopter presents concerns because a plurality of helicopter rotor blades will rotate above most, if not all, of the upper surfaces of the helicopter. The plurality of helicopter rotor blades will recurringly block the upper surfaces of the helicopter from orbiting satellites above the helicopter. Thus, if a microwave antenna is mounted on an upper surface of the helicopter, the plurality of helicopter rotor blades will repeatedly block a communication line of sight, path between the microwave antenna and the orbiting satellites.
When the microwave antenna is receiving a signal from the satellite, the plurality of helicopter rotor blades may block some sections of the signal in the time domain, and the receiver may have to request retransmission of those sections of the signal. When the microwave antenna is transmitting a signal, the problem is even more significant. Not only may the plurality of helicopter rotor blades prevent successful transmission of some sections of the signal, but the plurality of helicopter rotor blades also will reflect and scatter some fraction of the signal power in other directions, particularly downward. The reflected microwave signal presents a potential health concern for personnel working on or near the helicopter. The reflected microwave signals also may potentially interfere with the successful operation of other equipment on board the helicopter, or of electronic equipment located on the ground below the helicopter. Moreover, reflected and scattered energy from the reflected microwave signals will tend to make the helicopter electronically-visible to electronic signal detection equipment, putting the helicopter and its crew at risk of exposure and possible attack.
To circumvent the problem posed by mounting the microwave antenna on the upper surface of the helicopter, the microwave antenna can be mounted in other positions to avoid blockage by the plurality of helicopter rotor blades. For one example, the helicopter can be equipped with a mast extending beyond the area blocked by the plurality of helicopter rotor blades to prevent blockage of the communication line of sight path between the microwave antenna and the satellite. Unfortunately, the extent of the rotor sweep would, in almost all cases, leave little, if any room for a fixed mast. One alternative is to locate the microwave antenna above the rotor hub, as is done, for example, with the AH-64D Apache Longbow radar system. However, adding a rotor-mounted antenna to a helicopter would involve major modifications to the helicopter.
Alternatively, the microwave antenna may be mounted on a lateral surface of the helicopter, to transmit signals along a communication line of sight path that will not be blocked by the plurality of helicopter rotor blades. However, in order to provide continuous communications capability with the helicopter for all heading orientations, microwave antennas would have to be mounted on both sides of the helicopter. Even more problematic is that satellites would have to be orbitally-located such that at least two satellites would be in communications line of sight with the side-mounted microwave antennas. The need for additional satellites visible to the helicopter to facilitate such communication represents a substantial additional cost, and operational constraint in terms of the locations in which communications are possible.